The West African Monsoon Onset: a concise comparison of definitions

The onset of the West African Monsoon (WAM) marks a vital time for local and regional stakeholders. Whilst the seasonal progression of monsoon winds and the related migration of precipitation from the Guinea Coast towards the Soudan/Sahel is apparent, there exist contrasting man-made definitions of what the WAM onset means. Broadly speaking, onset can be analyzed regionally, locally or over a designated intermediate scale. There are at least eighteen distinct definitions of the WAM onset in publication with little work done on comparing observed onset from different definitions or comparing onset realizations across different datasets and resolutions. Here, nine definitions have been calculated using multiple datasets of different metrics at different resolution. It is found that mean regional onset dates are consistent across multiple datasets and different definitions. There is low inter-annual variability in regional onset suggesting that regional seasonal forecasting of the onset provides few benefits over climatology. In contrast, local onsets show high spatial, inter-annual and inter-definition variability. Furthermore it is found that there is little correlation between local onset dates and regional onset dates across West Africa implying a disharmony between regional measures of onset and the experience on a local scale. The results of this study show that evaluation of seasonal monsoon onset forecasts is far from straightforward. Given a seasonal forecasting model, it is possible to simultaneously have a good and bad prediction of monsoon onset simply through selection of onset definition and observational dataset used for comparison.